Acquisition and Divestiture Activity - Summary of Unaudited Pro-Forma Information (Detail) - Anchor Bank (MN) [Member] - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Business Acquisition [Line Items]
Revenue	$ 700,944	$ 738,280
Income before income taxes	$ 213,932	$ 214,737

The pro forma revenue for a period as if the business combination or combinations had been completed at the beginning of the period.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (h)(2)-(3)
-URI http://asc.fasb.org/extlink&oid=79982066&loc=d3e1392-128463

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration